Four different topics have been studied under this project: (a) (Eisenberg, Hill, Chen) An extensive study has been made of a four-state, single-site model of muscle contraction (recently submitted to Biophys. J.). This model represents both isometric transient and steady contraction data very well. Moreover, it conforms with current thinking on the biochemistry and kinetics of myosin-actin-ATP systems in solution. (b) (Hill, Stein, Chen) Our recent theoretical work on the influence of nearest-neighbor interactions on steady-state enzyme kinetics has been extended considerably. We have: considered small systems (enzymes lattices) with some empty sites and diffusions; studied the accuracy of the mean field approximation by making Monte Carlo calculations on the same systems; and investigated steady-state phase transitions that involve not only a conventional change in state but also a change in the dominant cycle of the kinetic diagram.